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BIG PHYSICS, BIG QUESTIONS –

Moon’s magnetic field lasted twice as long as we thought it did

The moon’s magnetic field came from its molten metal core

Hernán Cañellas

By Leah Crane

The moon may have had a magnetic field for at least a billion years longer than we thought. A moon rock brought back to Earth by Apollo 15 astronauts in 1971 shows the signature of a magnetic field that would have encompassed the moon more than 1 billion years ago.

When hot magnetic rocks are placed in the presence of a magnetic field, their electrons align with the field like tiny compasses. As the rocks cool, the orientations of those electrons, and thus the magnetic field, are preserved in stone.

Sonia Tikoo at Rutgers University in New Jersey and her colleagues reheated the Apollo sample to 780°C, around the temperatures in which it formed and exposed it to a new magnetic field. The rock was formed between 1 and 2.5 billion years ago, so it contains a record of the magnetic field that would have existed at that time.

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They found that the moon rock was formed in a magnetic field with a strength of about 5 microtesla, or about 10 times weaker than Earth’s current magnetic field.

Two periods of magnetism

Previous studies of moon rocks that formed 4 billion years ago showed that at that time the moon had a magnetic field with a strength of about 100 microtesla – even stronger than Earth’s current magnetic field. Around 3 billion years ago, that magnetic field died off.

Some researchers have proposed that the moon’s earlier magnetic field may have arisen through interactions with Earth’s gravity, since it orbited much closer to Earth 4 billion years ago than it does now. Back then, it was close enough for Earth’s gravity to pull and rotate the moon’s rocky exterior, dragging its molten center in the same direction. The motion of the liquid metals in the moon’s core would have created a strong magnetic field.

That original stronger magnetic field would have died off by 3 billion years ago as the moon drifted farther from Earth. At that point, a different mechanism must have created the moon’s new, weaker field, which stuck around for another billion years or more.

The researchers say that this field may have been sustained by the churning of the moon’s liquid core as it cooled and lower density molten rocks floated away from the centre.

“If there was a magnetic field on this moon that we’ve considered geologically torpid for so long, it might mean something for global magnetic fields on other planets,” says Georgiana Kramer at the Lunar and Planetary Institute in Texas.

Law of attraction

Right now, we don’t know why Mercury has a magnetic field while Mars and Venus don’t, Kramer says. Since the moon’s magnetic field may have been caused by its interaction with Earth, this sort of attraction between neighboring bodies may be important for maintaining a global magnetic field.

“Mars doesn’t have a nearby body that it could have these tidal effects with, but Mercury kind of does because of the sun,” says Kramer.

Since magnetic fields protect planets from charged particles from the sun, understanding which bodies do and do not have them could help us characterise the potential habitability of other worlds.